I have a 1600VA Multiplus on a vehicle with a 20ma RCD on the AC output of my Multiplus.
With this configuration, I don't understand the point of using a ground relay when the AC input is disconnected. To explain my point of view, I hypothesize that there is a malfunction in my installation and that the phase L touches the bodywork of my vehicle.
Option 1: Ground Relay Off
During a malfunction of this type, the most problematic in this configuration without ground relay is that the bodywork of the vehicle could be connected to the AC phase of the Multiplus without causing a stoppage of the Multiplus. In this case, if a person touches the bodywork and he is inside the vehicle, he is then touching the phase but the current has no way of returning to neutral, so no problem. If a person touches the bodywork and they are outside the vehicle, then the current could pass through it and go directly to earth, thus causing an imbalance between phase and neutral and the RCD will then trip to protect the person.
Option 2: Ground Relay On
If the phase of the Multiplus connects to the bodywork, phase and neutral will be connected together causing the immediate tripping of the Multiplus with a short circuit. This option is more secure because it immediately causes a stoppage of the Multiplus.
On the other hand, there is another hypothesis where the option 2 can be a problem in my opinion. If a person touches the output phase of the Multiplus (example of an intervention on a AC power socket in the vehicle) and this person is also in contact with the bodywork of the vehicle, in this case there is a danger. Indeed, the current will pass from the phase in its body will arrive on the bodywork connected to the neutral and the RCD will not detect any imbalance. The person will then transform into an electrical charge without there being a tripping of the RCD or Multiplus.
For my opinion, the option 1, RCD and no ground relay protects in all scenarios while option 2, RCD and ground relay may cause security problems in the last scenario above.
Thanks for your help